Electric power system for aircraft



Jan. 19, 1954 B. o. AUSTIN ETAL 2,666,871

ELECTRIC POWER SYSTEM FOR AIRCRAFT Filed July 15, 1950 Generator Unit minusssizs: INVENi'ORS 1% Boscum 0. Austin and Rol h D.Jessee. A BY ATTORNEY Fig.l.

Patented Jan. 19, 1954 UNITED STATES PATENT OFFICE ELECTRIC POWER SYSTEM FOR- AIRCRAFT Pennsylvania Application July 15, 1950, Serial No. 174,040

8 Claims.

Our invention relates, generally, to auxiliary electric power systems for aircraft and, more particularly, to a system having a plurality of enginedriven direct-current generators for supplying power to a load bus, such as the system disclosed in the copending application of B. 0. Austin, 0. C. Walley and D. W. Exner, Serial No. 12,118, filed February 28, 1948, now Patent No. 2,534,895, issued December 19, 1950.

An object of our invention, generally stated, is to provide a control and protective system for aircraft generators which shall be simple and efficient in operation and which may be economically manufactured and installed.

A more specific object of our invention is to simplify and improve the system described in the aforesaid copending application.

Another object of our invention is to provide a differential protective system for each one of a plurality of direct-current generators which are connected to a common bus.

A further object of our invention is to provide temperature compensation for the differential protective system for direct-current generators.

Still another object of our invention is to control the operation of dual switches in a generator circuit to secure back-up protection against reverse current in the generator and failure of one switch to open the generator circuit.

A further object of our invention is to protect against loose or broken connections in the generator circuits.

Other objects of our invention will be explained fully hereinafter or will be apparent to those skilled in the art.

In accordance with one embodiment of our invention, a T-connected shunt is provided in the positive leads for each generator in a power system having two or more direct-current generators connected in parallel-circuit relation to a load or load bus and the actuating coils of a differential relay are so connected to this shunt and the series field winding of the generator that protection is afforded against faults in the generator or its leads. Temperature compensation is provided to maintain a uniform value of tripping current for the differential relay throughout the wide range of temperature variations encountered in aircraft operation. Each generator is connected to the main bus through a pair of switches or contactors, the operation of which is so controlled that hack-up protection is afforded against reverse current and failure of one switch to open the generator circuit.

For a better understanding of. the nature and objects of our invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawing, in

which:

Figure 1 is a diagrammatic view of an auxiliary electric power supply and control system embodying the principal features of our invention, and

Fig. 2 is a detail view of a modification of a portion of the system shown in Fig. 1.

Referring to the drawing, the power system shown therein comprises a pair of generator units l5 and l l which are disposed to be connected in parallel-circuit relation between a power or load bus l2 and ground [3. More than two generator units may be employed if desired. Since the generator units are similar in construction and operation, only one unit is shown and described in detail in the present application.

The unit l0 comprises a direct-current generator It having an armature winding l5, a shunt field winding 16 and a series field winding ll. The generator may be driven by one of the engines (not shown) of an airplane in the usual manner.

The positive terminal of the generator H!- is connected to the load bus !2 through a feeder circuit 18 which comprises a generator lead I9, two parallel-connected feeder conductors 2| and 22 which are connected to the outside terminals of a T-connected shunt 23, a conductor 24 which is connected to the middle terminal of the shunt 23, contact members Cl 1 and CH of switches C! and C2, respectively, and conductor 25. The neg ative terminal of the generator is connected to ground I3 through a lead 26, and the series field winding 17.

As shown, the contact members CH and C2! of the switches Cl and C2 respectively are connected in series-circuit relation in the feeder circuit l8. As will be described more fully hereinafter, the operation of the switches Cl and C2 is so controlled by relays provided in the system that the contact members C2! are normally opened first to interrupt the generator circuit, and the contact members Cl l are opened follow ing the operation of the switch 02, thereby providing back-up protection against reverse current in the generator and failure of the one contactor to open the circuit.

A voltage regulator VB is provided for maintaining a substantially constant generator voltage. The regulator VR controls the energization of the shunt field winding IS in a manner well known in the art. In this instance the regulator comprises two carbon piles or stacks 3! and 32,

resistors 33 and '54 which are connected in a Wheatstone bridge circuit. The carbon pile 3i be compressed by a solenoid mechanism 35 having a main coil 36, an auxiliary coil 3? and a paralleling coil 53 disposed to actuate a core 38. The carbon pile 32 may be compressed by a solenoid mechanism ii having a main coil 42, an auxiliary coil (is and a paralleling coil M disposed to actuate a core 45. I

As shown, the main coils 36 and 42 are connected in seriescircuit relation between the power conductor 26 and ground in series with an adjustable rheostat 46 and a resistor 41. Thus,

1656 coils are energized in accordance with the generator voltage. The auxiliary coilsa'l and 43 are connected in series-circuit relation between two terminals of the Wheatstone bridge comprising the carbon piles 3i and 32 and the resistors 33 and 34. The paralleling coils 3B andAd-are connected between the negative terminal of the generator is and an equalizer bus 48 through contact members as of an eqalizer disconnect relay EDR, and a coil 5| of an overvoltage relay 0V. A ballast lamp 5t may be connected across the coil 51 as shown. The functions of the equalizer disconnect relay and the overvoltage relay will be explained more fully hereinafter.

The circuit for the shunt field winding I6 of the generator extends from the power conductor 26 through contact members FR! of a field relay conductor 52, the carbon piles 3i and 32, or the resistors 3.3 and at of the voltage regulator VR, conductor 53, the field winding l6, a conductor M, a portion ofthe series field winding I! and the conductor 26 to the negative terminal of the generator.

It will be understood that all of the regulators associated with the several generators connected to the bus l2 are so connected to their respective generators and to the common equalizer bus 48 that the main coils of the regulators function to cause the regulators to maintain the desired generator voltage, and the paralleling coils cause the regulators to maintain the desired load division between the generators.

As mentioned hereinbefore, the circuit for the shunt field winding 16 of the generator is controlled by the contact members FRI of the field relay FR. As shown, this relay is of a type which is latched in its closed position. provided with a reset or actuating coil FRC for actuating the relay to its latched position, and a trip coil for actuating the tripping mechanism of the relay.

A lockout relay L0, and a manually operable switch SWl are provided for retaining the field relay in its tripped position until it is reset by actuating the reset switch SW1. Thus, it is necessary to reset the relay FR after it has once been tripped before the generator can be put back in operation. An indicating lamp 55, which is energized through contact members PR2 of the relay FR is provided for indicating when the relay is in its closed position.

In addition to controlling the energizing circuit for the field winding it of the generator, the relay FR also functions to partially establish the energizing circuit for the actuating coils of the switches Cl and C2 which connect the generator to the load bus 22, as previously explained. Thus, whenever the field relay FR is tripped, the generator is disconnected from the load bus and its field circuit is opened.

In order to protect against overvoltage conditions, each generator is provided. with the over- The relay is all) voltage relay 0V which is connected to be responsive to the generator voltage. In addition to the coil 5|, previously mentioned, the relay 0V is provided with a coil 56, which is connected between ground and the conductor 52 through an adjustable rheostat 5'. and either through contact members 55 of the relay EDR to the conductor 52, or through contact members 53 of the relay EDR, conductor 6!, contact members of a relay PDR, conductor 82, contact members of a relay BUP, conductor 63, and a manually operable switch-SW3 to the conductor 52. As described in a copending application of J. D. Miner, B. 0. Austin and D. Jessee Serial No. 179,055, filed Augustx12, 1950, now Patent No. 2,590,265, issued March 25, 1952, the coil iii is energized by the equalizer current toenable the relays OV for the several generators to select a particular generator which is causing system overvoltage and sheet its disconnection from the bus.

The contact members of the relay 0V establish an energizing circuit for the trip coil FRT of the field relay FR. when the relay 0V is closed. The circuit for the trip coil extends from a control powerbusfi l through conductor 65, the contact members of the relay 0V, conductor 66, contact members PR3 and the trip coil FRT to ground.

It will be noted that the trip coil F tT may also be energized by closing a manually operable switch SW2. Thus, the generator is may be removed from service at any time during normal operation if desired by closing the switch SW2. Likewise, any other generator in the power system maybe removed from service by closing acorresponding switch.

Each generator in thesystem is also protected against reverse current flowing through the generator. In case of persistent overvoltage of any one generator in the system, reverse current will fiow through those. generators which are being regulated for normal voltage. Polarized relays PDR and BUP are provided for protecting the generator it against reverse current.

The relay PDR is provided with a re erse current coil PDRI, which is connected across a portionof theseries field winding ll by means of conductor 54. The relay is provided with a dif ferential voltage coil PDR2 which is connected between the generator power conductor 24; and the load bus l2 through a circuit which extends from the conductor 2:3 through contact members FR-i of the relay FR, conductor 6?, a coil BUPl on the relay BUP, conductor 63, the coil PDR2, conductor 69, the contact members of a relay PVR, and conductor 25 to the load bus Hi. The relay PDR is provided with a holding coil PDR3 which is energized through a circuit extending from the conductor 6i through the coil PDRS, conductor. H and the coil of the switch C2 to ground. The contact members of the relay PDP, are connected in the energizing circuit of the operating coil of the switch 02. The reverse current, coil PDR! is energized in response to the currentin the generator series field winding I1,

and reversal of the direction of current fiow through the generator, therefore, reverses the direction of energization of the coil PDR! and causes it to open the contacts of the relay, thus deenergizing the coil of the switch and effecting disconnection of the generator from the bus.

'I'herelay BUP functions to control the operationof the switch CI to provide back-up protection for the generator in case the switch C2 fails to open upon a reversal of generator current. In

nected to the middle terminal.

addition to the .coil :13. ll previously mentioned,

theurelay BUP'is provided with .another coil 'BUPZ "which is connectedacross one part of the T-connectedishunt 23 through conductors l2 and E3.

TherelayBUP is preferably adjusted to operate on a higher current setting than the relay PDR.

Asishown, the contact members ofthe relay IBUP' are connected, in the energizing'circuit for .both theswitch Ci and'the switch C2. Thecontact members of the relay PDB are. connected in the circuit for the switch C2 only. The relays are adjusted so that the contact members ofthe relay PDR open first to cause the switch C2 to open. If the switch 02 fails to open, the-contact members of the relay EU? will open, thereby causing the switch Ci to opento disconnect the generator from the load circuit.

In order to provide for connecting the genera tor Hi to the bus !2 only when its voltage is of proper polarity and value, the polarized voltage relay PVR, is provided to cooperate with the polarized differential relay FDR in controlling the operation of the switch C2. The coil of the relay PVR is connected between ground and the positive terminal of the generator lithrough conductor 53, the switch SW3, conductor 52, and contact members FRI of the relay FR to the positive power conductor 25. Thus, this relay is responsive to a generator voltage of predetermined magnitude and polarity and the closing which is connected between the power conductor '34 and the load bus 12 through the contact members of the relay PVR. Thus, the coil'PDRd is energized in accordance with the difference between the voltage of the bus i2 and the generator M. Accordingly, the contactmeinbers of'the relay PDR cannot close to cause the closing of the switch C2 until the generator voltage'is substantially equal to or greater than the voltage of the load bus i2, and of the correct polarity.

In order to protect the system against faults, such as grounds in the generator, or the generator leads and the feeder circuit, the r connected shunt 23 and a differential relay DP are provided. As previously explained, the shunt 23 is connected in the feeder circuit 18 between the generator I12- and the load bus 12. Feeder conductors 2i and 22 are connected to two outside'terminals of the shunt, and the conductor as is con- The relay DP is provided with a coil DP! which is connected between one outside terminal and. the middle terminal of the shunt 23. Another coil DP2 is connected between the other outside terminal and the middle terminal of the shunt. An adjustable rheostat it is connected in the circuit for the coils DP! and D92. A third .coil DPS connected across the series field winding ll of the generator M.

The coils BBQ and DB2 .are connected cumulatively and in opposition to the coil D1 3. Since the currents in the two partsof the shuntt i are equal, in normal operation, and thesum of these currents is equal to the current in the field winding H, the effect of this-arrangement isuthat the current in the reader conductor 2.! .-is balanced against thecurrent .inthe feeder conductor 22, and the sum of theseitwoicurrents.mbalanced against the current in the series field winding ii.

Bolilongasiche:respectiveJcurrents remain equal, thetrelay .DP is..not operated. A fault in the field circuit and cause the opening of switches Cl andC2 to disconnect the generator from the load bus 12.

In order to compensate for the wide variations in temperature-encountered in airplane operation, a length of wire 15 of a material having a high temperature coeflicient of resistance, such as pure iron or nickel, is connected in the circuit for-the relay coil DP3. The wire it is disposed in close proximity to the series field winding H, .as by winding the Wire along with the series winding 11.

Theohmicvailue of the wire iii'is such that any change-in temperature due to heat from the series field winding of the generator will compensate for the change in voltage across the series winding resulting'from the change in temperature of the generator winding. In other words, the resistance-of the wire plus the resistance of the rent in the shunt 23, throughout the operating range of the generator.

The T-connected shunt 23 is preferably made of a material having a very low temperature coefhcientof resistance. The temperature com-- pensation provided maintains a uniform value =of'trippingcurrent throughout the temperature variations of the generator encountered in aircraft operation.

If-desired, a shunt is having a temperature coefiicient of'resistance which is substantially the same as that of the series winding I? of the generator may be utilized in place of the resistor "15. As shown in Fig. 2, the shunt H3 is connected in the circuit for the series field Winding i! and the relaycoil DPS is connected across the shunt. For normal operation and load currents, the increase in resistance of the shunt and of the generator series winding I! will be substantially the same because of heating of the shunt by the current-flowing through'it. This scheme of temperature compensation is particularly suitable for utilizati'on'wi'th generators which have previously'been built, thereby making it impractical to utilize the resistance wire iii for temperature compensation.

A relayLT'having its actuating coil connected across the'two outside terminals of the shunt 23 "is provided to aflord protection against loose terminal connections at either end of the feeder conductcrsZl and 22. The relay LT is responsive to an unbalance in the currents carried by the two feeder conductors, since the currents in the two'parts of the shunt 23 are normally equal and opposite, and its contact members are connected to energize the trip coil FRT of the relay FR when they are closed by operation of the relay.

control the connection of the paralleling coils 3B and 44 of the voltage regulators to the equalizer bus 48. This connection extends from the bus 48 through the conductor 16 and the contact members 49 of the relay EDR to the paralleling coils through a circuit previously traced. The actuating coil of the relay EDR is connected between ground and the conductor 52 by the switch SW3. The conductor 52 is connected to the positive power conductor 24 when the contact members FRI of the relay FR are closed. Thus, the coil of the relay EDR is energized by the generator voltage to close its contact members 49, and the relay is automatically opened when the generator is shut clown by the operation of the field relay FR.

The operation of the system may be briefly described as follows. Assuming that the generator I4 is being driven by an engine (not shown), the field relay FR may be closed by operating the switch SW! first to the reset position and then to the on position. When the switch SWI is actuated to the reset position, the coil of the lockout relay L is energized through a circuit which extends from the control bus 64 through the switch SWI, conductor ll, an interlock 18 on the trip mechanism of the relay FR, and the coil of the relay L0 to ground. The relay L0 is operated to close its contact members L0! and L02. A holding circuit for the coil of the relay is established through the contact members L02 and the interlock 18 on the relay FR.

When the switch SWI is operated to the on position, the closing coil PRC of the relay FR is energized through the contact members LC! of the relay L0. The energization of the coil FRC actuates the relay FR to its uppermost position and it is latched in this position by the latching mechanism. After the relay FR is latched in its uppermost position the switch SW1 may be returned to its middle position. The switch SW2 is open and the switch SW3 is closed, during normal operation.

As explained hereinbefore, the closing of the contact members FRI of the relay FR establishes,

an energizing circuit for the shunt field winding l6 of the generator I4, thereby permitting the voltage in the generator to build up. The voltage is controlled by the voltage regulator VR. Closing of the field relay contact FR4 connects the relay coils PDRZ and BUP! to the conductor 24" and, when the generator voltage is of proper magnitude and polarity, the relay PVR closes its contact and completes the circuit of the coils PDR2 and BUPI. When the generator voltage is equal,

to or slightly greater than the bus voltage the contact members of the relays PDR and BUP are closed to complete the energizing circuits for the switches C! and C2. These circuits extend from the conductor 52 through the switch SW3, conductor 63 and contact of relay BUP to the coil of the switch Cl, and through conductor 62, contact of relay PDR, holding coil PDR3, and conductor H to the coil of the switch C2. Thus, when the relays PDR and BUP close their contacts, the switches Cl and C2 are closed to conreason, the reverse current coil PDRl of the relay PDR causes the relay contact to open, interrupting the circuit of the coil of the switch C2 to open the switch and disconnect the generator from the bus. If the switch C2 fails to open, the coil BUP2 of the back-up relay BUP Opens the contact of that relay and thus eifects interruption of the circuits of the coils of both switches C1 and C2, so that switch Cl opens. Since the relay BUP is preferably set to operate at a higher current than the relay PDR, the relay BUP will normally operate only if the switch C2 fails to open on reverse current or fails to interrupt the current. Since operation of the relays PDR and BUP does not trip the field relay FR, the system remains in operative condition and the generator is automatically reconnected to the bus, in the manner previously described, as soon as its voltage again equals or exceeds the bus voltage.

As described hereinbef-ore, the field relay FR is tripped upon the occurrence of either an overvoltage condition which causes the operation of the relay 0V, or a fault in the generator or feeder circuits which causes the operation of the differential relay DP, or loose connections in the feeder circuits which cause the operation of the relay LT. When this occurs, the field relay FR. functions not only to interrupt the control circuit for the line switches CI and C2, but also to interrupt the energizing circuit for the generator field winding I6. This not only disconnects the generator from the bus, but also reduces its voltage to zero. The generator cannot be reconnected until after the field relay FR has been reset by operating the switch SWI in the manner previously described to energize the lockout relay LO which has been opened by the operation of the tripping mechanism FRT of the relay FR.

However, if the generator is disconnected from the bus by the opening of the switch SW3, which causes the opening of the switches Cl and C2, it may be reconnected to the bus by simply closing this switch without resetting the relay FR, which has remained in its closed position. Also, if the generator is at any time disconnected from the bus through the operation of the polarized differential relay PDR, in response to a reverse current, as previously described, it will be automatically reconnected to the bus by the relay PDR when the generator again develops suilicient voltage to prevent reverse current from flowing through the generator from the bus l2.

From the foregoing description, it is apparent that we have provided an auxiliary electric power system which is particularly suitable for utilization on aircraft, but which is not necessarily limited thereto. The system provides protection against faults in the generators or their connec* tions, and also protects against overvoltage and reverse current conditions. Temperature compensation is provided to secure proper operation of the system throughout a wide range in operating temperatures.

Since numerous changes may be made in the above-described construction and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A control and protective system for a directcurrent generator having a shunt field winding and a series field winding, said system including aces-pen 9, a feeder circuit for connecting the generator to a load bus, at least a part of said feeder circuit comprising two parallel-connected conductors, relay means for balancing the currents in said parallel-connected conductors against the cur-- rent in the generator series field winding, said relay means being actuated by unbalance of said currents, means for effecting deenergization ofthe generator shunt field Winding and disconnection of the generator from the bus in responseto actuation of the relay means, and temperaturecompensating means connected in the circuit ofthe relay means, said temperature-compensating means comprising resistance means having a re sistance which varies with temperature in aman her to effect compensation for change in temperature of the generator series 'field'winding:

2. A control and protective system fora directcurrent generator having a shunt field winding and a series field winding, said system including a feeder circuit for connecting the generator to a load bus, at least a part of said feeder circuit comprising two parallel-connected conductors, a relay having an operating coil responsive to the current in one of said parallel-connected conductors, an operating coil responsive to the current in the other of said parallel-connected conductors, and an operating coil responsive to, thecurrent in the generator series field winding, the last-mentioned coil opposing thev two first-mentioned coils, said relay being actuated by unbalance of said currents, means for effecting deenergization of the generator shunt field winding and disconnection of the generator from the bus in response to. actuation of, the relay, and temperature-compensating means connected in the circuit of the last-mentioned relay coil, said temperature-compensating means comprisin resistance means having a resistance which varies with temperature in a manner to effect compensation for change in temperature of the generator series field winding.

3. A control and protective system for a directcurrent generator having a shunt field winding and a series field winding, said system including a feeder circuit for connecting the generator to a load bus, at least a part of said feeder circuit comprising two parallel-connected conductors, a relay having an operating coil responsive to the current in one of said parallel-connected conductors, an operating coil responsive to the current in the other of said parallel-connected conductors, and an operating coil responsive to the current in the generator series field winding, the last-mentioned coil opposing the two first-mentioned coils, said relay being actuated by unbalance of said currents, means for effecting deenergization of the generator shunt field winding and disconnection of the generator from the bus in response to actuation of the relay, and a conductor having a high temperature coeflicient of resistance disposed in close physical association with the generator series field winding and connected in the circuit of the last-mentioned relay coil to compensate for change in temperature of the generator.

4. A control and protective system for a directcurrent generator having a shunt field winding and a series field winding, said system including a feeder circuit for connecting the generator to a load bus, at least a part of said feeder circuit comprising two parallel-connected conductors, a relay having an operating coil responsive to the current in one of said parallel-connected conductors, an operating coil responsive to the current in the other of said parallel-connected conductors, and an operating coil responsive to the current in the generator series field winding, the last-mentioned coil opposing the two first-mentioned coils, said relay being actuated by unbalance of said currents, means for eifecting deener gization of the generator shunt field winding and disconnection of the generator from the bus in response to actuation of the relay, and a shunt connected in series with the generator series field winding; said shunt having approximately the same temperature coefficient of resistance as the series field winding, and the last-mentioned relay coil being connected across the shunt.

5. A control and protective system for a directcurrent generator having a shunt field winding and a series field winding, said system including means for connecting the generator to a load bus, said connectingmeans includingtwo series-con nected electrically operated switches, a first polarized relay having a contact connected in the energizing circuit: of one, of said switches, said first relay having a coil connected to be responsive to current in the generator series field winding and beingadapted to actuate its contact to effect opening, of said one switch in response to reverse current in the generator, and a second polarized relay having a contact connected in the energizing circuits of both said switches, said second relay having a coil connected to be responsive to current flow in said connecting means between the generator and the bus and being adapted to actuate its contact to effect opening of both. switchesin response to. reverse current in said, connecting means, the second relay being adjusted to respond to a higher value of current than the first relay.

6. A controland protective systemlfor a directcurrent generator having a shunt field winding anda series fieldwinding, saidsystem including means for connecting the generator to a load bus, said connecting means including two seriesconnected electrically operated switches, a shunt in the connecting means between the generator and the switches, a first polarized relay having a contact connected in the energizing circuit of one of said switches and having a coil connected across at least a part of the generator series field winding, said first relay being adapted to actuate its contact to effect opening of said one switch in response to reverse current in the generator, and a second polarized relay having a contact connected in the energizing circuits of both said switches and having a coil connected across at least a part of said shunt, said second relay being adapted to actuate its contact to effect opening of both switches in response to reverse current in the shunt, the second relay being adjusted to respond to a higher value of current than the first relay.

'7. A control and protective system for a direct-current generator having a shunt field winding and a series field winding, said system ineluding a feeder circuit for connecting the generator to a load bus, at least a part of said feeder circuit comprising two parallel-connected conductors, two electrically operated switches connected in series between said parallel con-- ductors and the bus, a differential relay having an operating coil responsive to the current in one of said parallel conductors, an operating coil responsive to the current in the other of said parallel conductors, and an operating coil respcnsive to the current in the generator series field winding, the last-mentioned coil opposing the two first-mentioned coils, the diiferential relay being actuated by unbalance of said currents, means for effecting deenergization of the generator shunt field winding and opening of said switches in response to actuation of said differential relay, a first polarized relay having a contact connected in the energizing circuit of one of said switches, said first relay having a coil connected to be responsive to current in the generator series field winding and being adapted to actuate its contact to effect opening of said one switch in response to reverse current in the generator, and a second polarized relay having a contact connected in the energizing circuits of both said switches, said second relay having a coil connected to be responsive to current flow in said feeder circuit and being adapted to actuate its contact to effect opening of both switches in response to reverse current in the feeder circuit, the second relay being adjusted to respond to a higher value of current than the first relay.

8. A control and protective system for a direct-current generator having a shunt field winding and a series fieldwinding, said system including a feeder circuit for connecting the generator to a load bus, at least a part of said feeder circuit comprising two parallel-connected conductors, two electrically operated switches connected in series between said parallel conductors and the bus, a field relay for controlling the energization of the generator shunt field winding and the energizing circuits of said switches, a differential relay having an operating coil responsive to the current in one of said parallel conductors, an operating coil responsive to the current in the other of said parallel conductors,

and an operating coil responsive to the current in the generator series field winding, the last-- mentioned coil opposing the two first-mentioned coils, the differential relay being actuated by unbalance of said currents, means for actuating 12 said field relay to effect deenergization of the generator shunt field winding and opening of the switches in response to actuation of said differential relay, a first polarized relay having a contact connected in the energizing circuit of one of said switches, said first relay having a coil connected to be responsive to current in the generator series field winding and being adapted to actuate its contact to effect opening of said one switch in response to reverse current in the generator, and a second polarized relay having a contact connected in the energizing circuits of both said switches, said second relay having a coil connected to be responsive to current fiow in said feeder circuit and being adapted to actuate its contact to effect opening of both switches in response to reverse current in the feeder circuit, the second relay being adjusted to respond to a higher value of current than the first relay.

BASCUM O. AUSTIN.

RALPH D. JESSEE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,008,511 McNairy July 16, 1935 2,056,040 Dozler Sept. 29, 1936 2,534,895 Austin Dec, 19, 1950 FOREIGN PATENTS Number Country Date 19,285 Great Britain Sept. 23, 1905 OTHER REFERENCES D.-C. 24-v01t Aircraft Electrical Systems, Westinghouse Engineer, Sept. 1950, page 212.

Electric-Circuit Fault-Protective Principles as Applied to D.-C. Aircraft Systems, AIEE, paper 44-38. 

